The source of the problem is in bold. Additionally, it also appears that the website's domain name is being appended to the IPv4 address, for whatever reason.

The normal solution to this problem is to replace the IPv4 address with a domain name, allowing things to work with NAT64 connections. It appears, though, that there's something weird going on with how these image URLs are being generated.

Questions are welcome.

EDIT: Linked stylesheets on the main page are also broken, as they are referenced using IPv4 literals.